Fusion down-draw process is used to make sheet glass. This process naturally produces sheet glass with pristine surfaces of fire-polished quality, and as such is valuable for making glass substrates for display applications and other applications requiring high-quality glass surfaces. The basic fusion down-draw process is described in U.S. Pat. No. 3,338,696 (published 29 Aug. 1967; Dockerty) and U.S. Pat. No. 3,682,609 (published 8 Aug. 1972; Dockerty). In general, the fusion down-draw process involves delivering molten glass to a weir of a fusion isopipe. The molten glass is allowed to overflow the top of the weir, where the overflowing molten glass divides into two separate streams that flow down opposite converging sidewalls of the fusion isopipe. The divided streams merge into a single stream at the root of the fusion isopipe. The single stream is then drawn down into a sheet glass. The inner surfaces of the divided streams that contact the sidewalls of the fusion isopipe end up in the interior of the sheet glass, while the outer surfaces of the divided streams that do not contact the sidewalls of the fusion isopipe end up on the exterior of the sheet glass, thereby endowing the sheet glass with the pristine surfaces of fire-polished quality.
Delivering the molten glass to the weir of the fusion isopipe is not a trivial matter because the quality of the sheet glass produced by the fusion draw process is directly dependent on the quality of the molten glass delivered to the fusion isopipe. Features such as inhomogeneity of the molten glass, both in terms of composition and temperature distribution, and solid or gas inclusions in the molten glass are not desirable. For production of large-scale commercial glasses, the delivery system usually includes a melting furnace, a refining furnace, and a stirring furnace. A glass batch is received in the melting furnace and melted to produce molten glass. Typically, gas is burned to provide the heat for melting the glass batch. The molten glass is continuously drawn from the melting furnace into the refining furnace, where gas inclusions are removed from the molten glass. The molten glass is then stirred in the stirring furnace to improve its homogeneity and delivered to the inlet of the fusion pipe, typically through an arrangement of a delivery pipe and downcomer.
The melting furnace is perhaps the most critical of the three furnaces in the delivery system since it is in the melting furnace that the molten glass is produced. While melting the glass batch in the melting furnace, it is important to monitor conditions within the melting furnace. The results of the monitoring may be used to adjust the operation of the melting furnace so that the melting furnace operates efficiently and produces high-quality molten glass. Examples of conditions that could be monitored are temperature changes, molten glass flow patterns, molten glass/glass batch interface, foam production, foam/glass batch interface, and refractory outgassing. Video technologies using charge-coupled device (“CCD”) or complementary metal-oxide-semiconductor (“CMOS”) sensors have been used to capture images of the interior of a melting furnace. However, these video technologies have been unable to produce clean images in the presence of gas or foam within the melting furnace. The visible wavelengths (400-650 nm) used in these video technologies cannot penetrate the gas; therefore it is not possible to see behind the gas flames. Also, the video sensors are insensitive to temperature and not useful in thermal mapping of the melting furnace. Instead, thermocouples on the bottom of the melting furnace and thermal output of the burners at the crown of the melting furnace are being relied upon to assess temperature distribution within the melting furnace.